The Required Minimum Distribution (RMD) amount is taxed the same other ordinary income. There are a lot of different components to a tax return, so it's impossible to say what's causing what you're seeing with very limited information.
One common reason is that the additional taxable income from the retirement distribution you entered can be causing an additional amount of your Social Security income to become taxable. This means that even though the retirement income falls in the 25% tax bracket, the marginal tax rate is somewhat higher. Check to see how the amount on Form 1040 line 20b or Form 1040A line 14b is affected by the additional retirement income.
Increases in your AGI can also affect various deductions and tax credits, so there are other possibilities in addition to the effect on the taxability of Social Security income. Review the entire Form 1040 or 1040A for related changes, not just the lines where the retirement income appears. Alternative Minimum Tax is another possibility.
Related information:
